This Hypselodoris individual is juvenile so I cannot decide whether it is Hypselodoris nigrostriata (because of the diagonal lines across the mantle) or Hypselodoris zephyra (because of the lack of a distinct yellow margin). That is assuming they are different species – as I have said several times before I think they are just colour forms of the same species.

The striped Hypselodoris in your photo is Hypselodoris nigrostriata because the lines are more or less oblique. In the very similar species, Hypselodoris zephyra the lines are parallel. I thought they were the same species, but then Rudie Kuiter showed me a lot of his pictures from Bali where both species seem to occur together and he has made a strong argument that they are separate in the new book Nudibranchs of the World. So I haven’t made up my mind yet. One other character supposedly only possessed by Hypselodoris nigrostriata is a yellow border to the mantle. I really cannot see this yellow band in the animal in your picture, so maybe that character is no good?

It is interesting to see an individual without any yellow spots between the obliquely-directed black lines. It rekindles the debate over whether Hypselodoris nigrostriata (with oblique lines) and Hypselodoris zephyra (with horizontal lines) are really different species.

Similar Species

Hypselodoris emmae: The body is a pale cream or yellow with a purple or bluish border to both the foot and the mantle. there are three purple longitudinal lines on the mantle and four brownish lines which vary in intensity in different individuals. The gills and rhinophores are a brilliant reddish orange and in Pacific specimens there is a white tip to the rhinophores.

Hypselodoris whitei has a whitish background color with a reddish purple submarginal line around the mantle edge and a series of five longitudinal lines covering the rest of the mantle. These lines are usually very crooked and joined by a series of lateral connections, breaking the mantle into rows of slightly raised whitish regions. The rhinophores are orange to orange-red with a distinctive white tip and the gills are similarly coloured with white on the inside and usually at the tip of each gill.

Hypselodoris maridadilus is yellow, with a purple border to the foot and the mantle, and a series of five longitudinal purple lines. the gills and rhinophores are bright orange-red.